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The behavior of concrete under high strain rates is often described by plasticity models with softening, which is modeled by a reduction of the yield surface as a function of the local equivalent plastic strain. Among these are the RHT model, the K\&C model and the Johnson-Holmquist concrete model. These models are however local and therefore produce mesh-dependent results.
In this contribution, the gradient-enhancement of such models is investigated. First, the mesh-dependency of these local formulations based on the analysis with a modified JH2 model as a representative for these constitutive formulations is demonstrated using a one-dimensional benchmark example. The central difference method is used as solver with a diagonal mass matrix obtained from a Gauß-Lobatto integration.
In the benchmark, the width of the damaged zone decreases upon mesh-refinement and the dissipated plastic energy tends to zero. It is further shown that a significantly small safety factor for the critical time step is needed in order to achieve accurate results for the benchmark example.
Next, two gradient-enhancement approaches are investigated. The enhancement is based on the inclusion of inertia and damping to the additional Helmholtz equation which enables the use of the central difference method as an explicit solver. In the first formulation, the yield surface and therefore the softening is formulated in terms of a nonlocal equivalent plastic strain. In the second approach, a hardening term which depends on the local equivalent plastic strain is introduced to the modified JH2 model in addition to the nonlocal softening. This approach is inspired by results from gradient plasticity in quasi-static loading scenarios. It is shown that the approach without hardening can still lead to mesh-dependent results while the model that includes hardening successfully inhibits strain localization and leads to a converging dissipated plastic energy. This is further confirmed in a two-dimensional wedge-splitting experiment where the damage pattern produced by the local model is mesh-dependent as well and the dissipated plastic energy tends to zero with mesh-refinement. The proposed nonlocal model with hardening results in a consistent damage pattern and the dissipated plastic energy converges. Furthermore, the nonlocal model with hardening is less sensitive to time step refinement, such that computational efficiency can be improved compared to the local model.
The numerical experiments are implemented using the free and open-source tool FEniCSx.
The behavior of high‐density polyethylene with respect to resistance against environmental stress cracking (ESC) is usually regarded as an inherent material property being specific for respective types of PE‐HD and tested using standardized methods, conditions, and also standard testing liquids (usually aqueous surfactant solutions). On the other hand, for practical applications the ESC behavior of those polymeric materials, commonly used for pipes or containers, in contact with other liquids (e.g., fuels) is often of relevant interest, but for a reasonable assessment, where consistent benchmark data for a direct comparison are often missing, it is essential to determine the actually prevailing failure mode and classify it related to crack propagation or other mechanisms. Using the well‐established Full Notch Creep Test, which favorably allows for a detailed microscopic fracture surface analysis after failure, the behavior of two typical PE‐HD types for container applications is investigated in biodiesel and diesel and compared to a standard surfactant solution. This enables a clear identification of characteristic features of the interaction of biodiesel and diesel as sorptive fuels in contact with the polymer, revealing the complex interplay of sorption and plasticization as well as ESC inducing effects on PE‐HD, which could be clearly shown for both fuels.
Die Wechselwirkung von hoher Alkalität und feuerverzinktem Betonstahl in der Frischbetonphase generiert einen Störung in der Verbundzone. diese Störung basiert sowohl auf der Wasserstoffentwicklung in der Frischbetonphase bis zur diffusionskontrollierten Decksschichtbildung als auch auf die erstarrungsverzögernde Wirkung von Zinkionen auf die CSH-Phasenbildung. Diese werden zwangsläufig infolge anfänglicher starker Korrosionsprozesse frei, und reichern sich in der Verbundzone an. Hier kann es beim normgerechten Ausschalen nach 24h zu weiteren Störungen des Haftverbundes kommen das davon Auszugehen ist das die Erstarrung der CSH-Phasen in der Verbundzone noch nicht begonnen hat.
Kurzvorstellung zu einem DFG-Antrag. Themengebiet sind alterative Bindemittel für die Herstellung von Stahlbetonbauwerken in Kombination mit feuerverzinktem Betonstahl. Die Möglichkeit Bindemittel zu verwenden die keine Alkalitätsreserve vorhalten müssen lässt sich mit feuerverzinktem Betonstahl generieren. Hierbei können die pH-Werte um 8 mit einer Reduktion der Korrosionsgeschwindigkeit einhergehen, sofern man den direkten Vergleich mit hochalkalischen Betonen anstrebt.
This research seeks to understand and identify the most promising approaches to enhance the flame resistance of poly(limonene carbonate) (PLimC). Furthermore, the goal is to develop a PLimC-based material that incorporates halogen-free flame retardants (FRs) that are not only highly effective but also environmentally sustainable, contributing to the advancement of greener materials for safer use.
The plastic industry depends on fossil-based materials, causing environmental concerns. PLimC, a sustainable polymer derived from limonene and CO₂, offers a promising alternative. Its use in fire-safe applications underscores the need to optimize its performance. This study evaluates halogen-free FRs to identify and understand the most promising approaches to enhance PLimC’s flame resistance, aiming to improve fire safety and support eco-friendly material development.
This study has as its main task to conduct experimental evaluations to analyze the thermal properties, flammability, and fire behavior of each system. Pyrolysis, together with the evolved gases of the samples, were analyzed by thermogravimetric analysis coupled with a FTIR spectrometer. The energy content was determined by using a bomb calorimeter. The flammability of the samples was assessed by the reaction to small flame tests such as the UL 94 burning chamber and limiting oxygen index (LOI). The burning behavior in forced flamed conditions was evaluated using the cone calorimeter.
Thermal properties were first evaluated to gain a deeper understanding of the pure PLimC thermal stability and fire behavior. This analysis builds on PLimC's structural similarity to polycarbonate (PC), due to its carbonate group, and to polyolefins (PO), due to its aliphatic limonene-derived segment, providing insights into optimizing PLimC for sustainable, fire-safe applications.
The initial results, which defined our starting point, showed that the LOI of PLimC was 17.2%, very close to that of PO such as polyethylene (PE) and polypropylene (PP) (~18%). In contrast, PC has a higher LOI of ~24%. Additionally, PLimC did not produce any char (0 wt.-%), similar to PO, whereas PC forms char due to its phenolic structures. The effective heat of combustion of PLimC, measured using bomb calorimetry, was determined to be 31.1 MJ/kg. This value is comparable to that of PC (~30 MJ/kg) but significantly lower than PO (~44 MJ/kg). These findings, which demonstrated similarities to both PO and PC, prompted the evaluation of various commercially available FRs at standard market concentrations.
Four halogen-free FR systems were chosen for evaluation: 1. mixture of APP (20 wt.-%) + pentaerythritol (10 wt.-%) as an intumescent system [PLimC / APP / Penta], 2. mixture of the phosphorus compound PX200® (16 wt.-%) + PTFE (0.4 wt.-%) as antidripping [PLimC / PhosC / PTFE], 3. aluminum trihydroxide ATH (50 wt.-%) [PLimC / ATH], and the potassium sulfonate salt Bayowet® (0.4 wt.-%) [PLimC / SulfS]. The concentrations were decided according to the standards used in the industry.
Cone calorimeter results show that PLimC has the usual behavior of a non-charring specimen (fast burning and high peak of heat release). ATH proved to be the most effective flame retardant, achieving the biggest reductions in the effective heat of combustion (EHC) and total heat evolved (THE). Additionally, ATH achieved the highest increase in the LOI (17.1% to 26.0%). With these results, we understood that the FRs commonly used with polyolefins (at concentrations standard in the industry) exhibit similar behavior in terms of flammability and flame retardancy when applied to PLimC.
Additionally, we aim to investigate bio-based flame retardants to further improve the sustainability of the material. Given that phytic acid salts and lignin have demonstrated effective flame-retardant performance in polyolefins, we consider these compounds promising candidates for application in our system.
Rather environmentally friendly flame retardant bio-composites have become part of the state of the art. Using two illuminating examples the talk gives an outlook towards sustainability beyond “partly bio-carbon”. In waste water phosphorous-enriched microalgae are introduced as bio-based adjuvant in polylactide (PLA). The flame retardancy effect is discussed and compared with other novel bio-based additives. In a combination with a commercial flame retardant, half of the commercial flame retardant can be replaced without worsening the UL 94 classification V0. In the second example flame retardant routes for poly(limonene carbonate) are discussed. Poly(limonene carbonate) is synthesized with a bio-based monomer limonene oxide and CO2. A novel bio-based flame retardant melem phytate together with bio-based charring agents is proposed to realize a fully bio-based flame retardant poly(limonene carbonate).
Die vorgestellten Ergebnisse eines gemeinsamen Forschungsvorhabens der BAM, Straßen.NRW, LSBG, ISER und weiteren Partnern zeigen geeignete Duplexstähle für die Betriebs- und Sicherheitsausstattung in Straßentunneln, die in einer Parameterstudie in mehreren Straßentunneln umfassend untersucht worden sind. Spaltkorrosion war dabei die maßgebliche Korrosionsart und wurde als Bewertungskriterium herangezogen. Die Werkstoffgruppe CRC III mit einem PRE(N)-Wert von 24 bis 27 zeigte in den Untersuchungen eine nur ungenügende Spaltkorrosionsbeständigkeit. Das für die Wahrscheinlichkeit der Spaltkorrosionsbeständigkeit anzustrebende 95 %-Quantil wird erst bei PRE(N) > 34 erreicht. Die Erkenntnisse aus der Parameterstudie wurden in einem Pilotprojekt von Straßen.NRW umgesetzt, bei dem unter anderem Strahlventilatoren vollständig aus Duplexstahl 1.4462 gefertigt worden sind. Dadurch soll eine Nutzungsdauer von mindestens 35 Jahren sichergestellt werden.
Die vorgestellten Ergebnisse eines gemeinsamen Forschungsvorhabens der BAM, Straßen.NRW, LSBG, ISER und weiteren Partnern zeigen geeignete Duplexstähle für die Betriebs- und Sicherheitsausstattung in Straßentunneln, die in einer Parameterstudie in mehreren Straßentunneln umfassend untersucht worden sind. Spaltkorrosion war dabei die maßgebliche Korrosionsart und wurde als Bewertungskriterium herangezogen. Die Werkstoffgruppe CRC III mit einem PRE(N)-Wert von 24 bis 27 zeigte in den Untersuchungen eine nur ungenügende Spaltkorrosionsbeständigkeit. Das für die Wahrscheinlichkeit der Spaltkorrosionsbeständigkeit anzustrebende 95 %-Quantil wird erst bei PRE(N) > 34 erreicht. Die Erkenntnisse aus der Parameterstudie wurden in einem Pilotprojekt von Straßen. NRW umgesetzt, bei dem unter anderem Strahlventilatoren vollständig aus Duplexstahl 1.4462 gefertigt worden sind. Dadurch soll eine Nutzungsdauer von mindestens 35 Jahren sichergestellt werden.
Implementation, data, and results for the generation of the study "Characterization of temperature influence on the structural built-up of 3D concrete".
In this study, the influence of temperature on structural build-up is investigated. A significant temperature influence is demonstrated for three experimental setups (small amplitude oscillatory shear, constant shear rate, and small amplitude oscillatory extensional tests) using different materials.
A common modeling framework capturing the time- and temperature evolution is derived based on the maturity approach. Two alternative formulations for the time evolution were proposed. Both models were calibrated using a probabilistic approach, allowing for uncertainty quantification. The calibrated models successfully predict the structural build-up under different ambient temperature conditions.